1. Field of the Invention
The present invention relates to an infrared sensor, an infrared camera, a method of driving an infrared sensor, and a method of driving an infrared camera.
2. Related Art
According to infrared imaging, it is possible to pick up images regardless of day or night, and an infrared ray has higher permeability of smoke and fog than that of visible light. Based on the infrared imaging, temperature information of a photogenic subject can be obtained. Therefore, infrared imaging has a wide application range using an infrared camera as a monitoring camera for military operations and as a fire detection camera.
A largest issue of a conventional quantum infrared solid-state imaging device is that the device requires a cooling mechanism for a low-temperature operation. In recent years, development of an “uncooled type infrared solid-state imaging device” that does not require this cooling mechanism is being progressed. The uncooled type, or heat-type, infrared solid-state imaging device converts an incident infrared ray, having a wavelength of about 10 microns, into heat in an absorption structure. After this, a certain thermoelectric converter converts a temperature change of a thermosensitive part, generated by weak heat, into an electric signal. The heat-type infrared solid-state imaging device reads out this electric signal to obtain infrared image information.
For example, an infrared sensor that uses a silicon pn junction which converts a temperature change into a voltage change, by giving a constant forward current, is reported (Tomohiro Ishikawa, et al. Proc. SPIE Vol. 3698, p. 556, 1999).
According to this system, an SOI (Silicon On Insulator) substrate is used for a semiconductor substrate. Because an infrared sensor can be manufactured in only a silicon LSI process, the infrared sensor is suitable for mass production.
The infrared sensor achieves a row selection function, by using rectification characteristic of a silicon pn junction as a thermoelectric conversion unit. Therefore, this infrared sensor also has a characteristic that a pixel configuration can be extremely simply structured.
As one of indexes that represent performance of the infrared sensor, there is NETD (Noise Equivalent Temperature Difference) which expresses temperature resolution of the infrared sensor.
The NETD shows a temperature difference that is equivalent to noise. Decreasing the NETD is important for the infrared sensor. For this purpose, it is necessary to increase the sensitivity of a signal and decrease noise.
A signal detected in an imaging area of the infrared sensor is amplified by a column amplifier, and the amplified signal is stored in a storage capacitor. Image information is obtained by reading the signal from the storage capacitor.
However, a signal that is detected in the imaging area in the actual infrared sensor is a very weak voltage in the order of micro volts. On the other hand, a voltage component necessary to provide a bias current for improvement of the gain of the column amplifier is a few hundred mV in a column amplifier input conversion. A bias current flows through an amplifying transistor of the column amplifier, regardless of presence or absence of an infrared signal.
As explained above, a most part of current stored in the storage capacitor is a bias current component, and a signal component of an infrared ray is very small. In other words, most of a voltage swing of the storage capacitor is a bias current component, and the infrared signal component is only a small part of the voltage swing.
In general, an external circuit removes the bias current component. Therefore, conventionally, a dynamic range of the column amplifier is not sufficiently used within the infrared sensor. Because a signal needs to be amplified after the external circuit removes the bias current component, a standard of noise of the external circuit other than the infrared sensor has had to be critical. Therefore, the cost of an electronic device including an infrared sensor such as an infrared camera has been high. The gain of the column amplifier cannot be sufficiently large, because the storage capacitor needs to be prevented from being saturated by a bias current component. As a technique of preventing the column amplifier from being saturated, JP-A H09-284651 (KOKAI) discloses a method of removing a current corresponding to the bias component by a MOS transistor that is drain connected to the storage capacitor.
However, a threshold value of the MOS transistor is known to have a fluctuation or variation about 30 mV. Precision of removal of the bias component is no more than about 30 mV in a column amplifier input conversion.
On the other hand, the signal component is very weak in the order of micro volts in the column amplifier input conversion, as described above. Therefore, a bias component of about 30 mV that cannot be removed remains. The gain of the column amplifier cannot be made sufficiently large. For the same reasons, the voltage swing of the column amplifier cannot be activated to the full.